1. Field of the Invention
The present invention relates to an elastic wave filter device used as, for example, an RF stage bandpass filter of a cell phone and, in particular, to a longitudinally coupled resonator type elastic wave filter device including first to fifth IDT electrodes.
2. Description of the Related Art
It is strongly desired that passband filters have a steep filter characteristic in a range between the passband and the stopband. Japanese Unexamined Patent Application Publication No. 4-54011 describes a surface acoustic wave filter device that increases the steepness of the filter characteristic. In the surface acoustic wave filter device described in Japanese Unexamined Patent Application Publication No. 4-54011, an elastic wave resonator is connected to a 3-IDT longitudinally coupled resonator type elastic wave filter device in series or in parallel. By controlling the locations of the resonance frequency and the anti-resonance frequency of the elastic wave resonator connected in series or in parallel, the steepness of the filter characteristic in the low frequency range and the high frequency range of the passband can be increased.
However, in such a method using the frequency characteristic of an elastic wave resonator, the steepness increasing effect depends on the Q value of the elastic wave resonator. The Q value of the elastic wave resonator is primarily determined by the material of a piezoelectric substrate and the material of an electrode. Accordingly, it is difficult to further increase the steepness of the filter characteristic by increasing the Q value of the elastic wave resonator.
On the other hand, WO2006/068086A1 describes a 5-IDT longitudinally coupled resonator type surface acoustic wave filter device having a balanced-unbalanced conversion function. FIG. 26 is a schematic plan view of the surface acoustic wave filter device described in WO2006/068086A1. A surface acoustic wave filter device 1001 includes a piezoelectric substrate 1002. As shown in FIG. 26, an electrode structure is provided on the piezoelectric substrate 1002. The electrode structure is connected between an unbalanced terminal 1003 and each of a first balanced terminal 1004 and a second balanced terminal 1005. A 5-IDT longitudinally coupled resonator type elastic wave filter portion 1010 is connected to the unbalanced terminal 1003. The 5-IDT longitudinally coupled resonator type elastic wave filter portion 1010 includes first to fifth IDT electrodes 1011 to 1015 and reflectors 1016 and 1017. In this example, the first to fifth IDT electrodes 1011 to 1015 include, on the end adjacent to a different IDT electrode, a narrow-pitch electrode finger portion N1011, a pair of electrode finger portions N1012a and N1012b, a pair of narrow-pitch electrode finger portions N1013a and N1013b, a pair of narrow-pitch electrode finger portions N1014a and N1014b, and a narrow-pitch electrode finger portion N1015, respectively.
In addition, surface acoustic wave resonators 1021 and 1022 are connected downstream of the 5-IDT longitudinally coupled resonator type elastic wave filter portion 1010.
In this example, in the narrow-pitch electrode finger portion N1011 of the first IDT electrode 1011 connected to the first balanced terminal 1004 and the narrow-pitch electrode finger portion N1015 of the fifth IDT electrode 1015 connected to the second balanced terminal 1005 and in narrow-pitch electrode finger portions N1013a and N1013b of the third IDT electrode 1013, the electrode finger pitch of the narrow-pitch electrode finger portion on the side at which the number of electrode fingers of the narrow-pitch electrode finger portion is greater, is greater than that on the side at which the number of electrode fingers of the narrow-pitch electrode finger portion is fewer. In this manner, a ripple is reduced in the passband. However, in such a structure, the steepness of the filter characteristic cannot be sufficiently increased.
In addition, WO2007/083503A1 describes a 5-IDT longitudinally coupled resonator type elastic wave filter device which does not include any narrow-pitch electrode finger portions. In this example, the number of electrode fingers and the pitch of the electrode fingers of an IDT electrode located in the middle are less than the number of electrode fingers and the pitch of the electrode fingers of an IDT electrode located on either side of the IDT electrode located in the middle. In this manner, the attenuation in the stopband adjacent to the low frequency range of the passband is increased and, therefore, the steepness of the filter characteristic is increased.
As noted above, the surface acoustic wave filter device described in Japanese Unexamined Patent Application Publication No. 4-54011 is limited in the amount that the steepness of the filter characteristic can be increased. Accordingly, the elastic wave filter device cannot provide a sufficiently increased steepness of the filter characteristic that has been required in recent years.
In contrast, the longitudinally coupled resonator type surface acoustic wave filter device described in WO2006/068086A1 only reduces a ripple that occurs in the passband by controlling the electrode finger pitch and the number of the electrode fingers in the narrow-pitch electrode finger portion in the above-described manner. In WO2006/068086A1, the structure that increases the steepness of the filter characteristic is not described.
In WO2007/083503A1, the longitudinally coupled resonator type surface acoustic wave filter device including no narrow-pitch electrode finger portions increases the steepness of the filter characteristic by controlling the number of electrode fingers and the electrode finger pitch of the IDT electrode located in the middle and the number of electrode fingers and the electrode finger pitch of an IDT electrode located on either side of the IDT electrode located in the middle. However, the steepness of the filter characteristic is still not sufficiently high and, therefore, there is a demand for the steepness to be further increased.